score:2
For itinerary management, I go with TripIt Pro that is by far the best at integrating reservation emails from various sources and keeping you up to date on changes to flights.
For finding local restaurants and places to visit, Trip Advisor is a good resource to have handy just about anywhere.
For hotels, well I recommend the Agoda.com app but as developing that is actually my job, you could consider that recommendation biased. But if anyone has a reason some other app is better, let me know and we'll fix that!
For free offline maps with navigation while traveling, I find MAPS.ME generally reliable ... but you're well advised to have a variety of mapping programs available if you're going outside major cities, as the quality and accuracy of their data vary wildly.
Getting to know other travelers, well personally I think the point of traveling is to get to know the locals so I'm not active on any, but if I was into that I'd probably pick Travello as they run contests for sharing content regularly -- right now there's one on to win a Parrot Bebop drone, which is an alright piece of kit.
One class of app I do like to travel with that most people overlook is astronomy apps, to figure out what's in the sky in strange places -- NightSky and Skyview are my choices there!
Upvote:3
There's no one answer for this, even though plenty are trying. Mostly, getting traction is difficult, while every individual's needs are just too different to easily define a lowest common denominator.
You should check out TripIt, designed to keep all your travel plans together, integrating with your email inbox, and its alternatives.
Also worth checking out is Esplorio.